Generally, light-emitting diode (LED) loads are driven by an electric current source due to the electrical characteristics and luminescence characteristics of LED. When an electric current source is configured for supplying electric power to LED loads, and if the LED loads malfunction, for example, if one or more LED loads in a LED loads string is damaged or if the whole LED loads string is in poor contact with the circuit, the LED loads would present an open circuit state. If no protecting measure is taken under the open circuit state, the output voltage would increase rapidly, and the rest parts of the circuit would be damaged. Therefore, an open circuit protecting function is very necessary.
For realizing the open circuit protecting function, there have been many open circuit protecting technologies, and in one of which a zener diode is anti-parallel connected to a plurality of series-connected LED loads. As shown in FIG. 1, when no LED load is open circuited in a branch of series-connected LED loads 4, an electric current from the electric current source passes through all the series-connected LED loads 4, and no zener diode 42 would be reversely brokedown. Or else, if one LED load in the branch of series-connected LED loads 4 is open circuited because of being damaged or in poor contact with the circuit, the zener diode 42 which is anti-parallel connected to the open circuited LED load 4 would be reversely brokedown, and a large part of the electric current would flow into the zener diode 42 which is anti-parallel connected to the open circuited LED load 4, thus the whole series-connected LED loads string would not be open circuited, so as to ensure that other LED loads can operate normally. Nevertheless, this technology would cause a relatively large power cost on the zener diode 42. Moreover, because the zener diodes 42 and the LED loads string are integrated together in this technology, the rest parts of the circuit may still be damaged if many LED loads in the whole branch are all in poor contact with the circuit or if there is no LED load connected into the branch.
Besides, silicon controlled rectifiers (SCRs) are connected in parallel with LED loads in another technology. As shown in FIG. 2, when a LED load 100 is not open circuited, the voltage of the control terminal of a SCR 120 is lower than the “threshold” voltage thereof, so an electric current from an electric current source passes through the LED load 100, and the SCR 120 presents an “off-state”. If the LED load 100 is open circuited, the voltage of the control terminal of the SCR 120 is larger than the “threshold” voltage thereof, so the SCR 120 is turned on and converted from the “off-state” to an “on-state”, the electric current can pass through the SCR 120, and the LED load is short circuited, i.e., the open circuit protection is realized for the LED load. This technology can protect the circuit from being destroyed. However, the SCR can not be turned off after being turned on as it is a half-controlled device. Therefore, when the LED load on the output side resumes to a normal state, the SCR cannot resume to the normal work state automatically, i.e., the LED load cannot be provided with a function of hot-plugging.
Above all, it is an effort in the direction for those skilled in the art to solve any one problem existed in the present open circuit protecting circuits.